Solari: Light leak reduction and stochastic WC update

[JMS55]

Objective

• Greatly reduce light leaks near corners of objects (but doesn't fix general light leaks from tiny geometry)

Solution

To prevent light leaks:

• When ray_t is smaller than the cache cell size, force the finest LOD cache access.
• Make LOD much less aggressive
• Make the finest LOD smaller

Since this then greatly increases the number of cache cells in the scene, I changed the following to prevent too much of a decrease in performance (this PR is still a hit to performance though):

• Randomly skip updating some cells once we have more than 40,000 cells active
• Aggressively prune older cache cells

Also some misc changes:

• Split the DI/GI WC sampling steps into separate dispatches (small performance win)
• Added a counter of current WC cell count to the example UI (using Add render diagnostic functions for reading from a buffer #22326)

Ideally the skipped cells wouldn't be totally random, and we'd have coarser-LOD cells update less frequently, but that would get very complicated as we'd have to track the total amount of cells per LOD in order to average out to WORLD_CACHE_TARGET_CELL_UPDATES.